1. Field of the Invention
This invention relates to the improvement of an etching treatment which is performed without heating a substrate using a plasma-treatment apparatus capable of stably inducing discharge under atmospheric pressure and obtaining low-temperature discharge plasma. Further, this invention relates to a plasma-treatment method in which kinds of gas usable for the plasma-treatment can be multiplied to perform discharge under various experimental conditions, and also to a plasma-treatment method in which the etching treatment can be stably performed irrespective of insulation property/resistant value of a substrate. Still further, this invention relates to a plasma-treatment apparatus capable of stably obtaining sheet-shaped (planar) and low-temperature discharge plasma under atmospheric pressure.
2. Description of Related Art
An atmospheric-pressure state serves as an insulator under low electric field, however, application of high electric field such as direct current, alternating current and impulse causes the insulator to be broken down, so that current flows through the insulator (self-maintaining discharge). The self-maintaining discharge is classified into a corona discharge, glow discharge and arc discharge. When uniform electric field is applied, all paths are broken down (all-path breakdown is induced) immediately after the atmospheric-pressure state is shifted to the self-maintaining discharge, and thus the self-maintaining discharge proceeds to the glow discharge or the arc discharge. On the other hand, when ununiform electric field is applied, breakdown is first locally induced at positions to which higher electric field is applied, and the corona discharge is induced at these portions. Thereafter, as the electric field is intensified, the all-line breakdown is induced. In an atmospheric-pressure air, the self-maintaining discharge is frequently directly shifted to the arc discharge without passing through the glow discharge when the state suffers the all-line breakdown. The reason for this would be considered as follows. The arc discharge is based on thermionic emission (existence of cathode spot) due to electrode heating which is caused by incident ions. The number of incident ions to the electrode under high pressure is higher than that under low pressure, and thus the electrode is heated for a very short time to induce thermionic emission. For current below 2A, the occurrence of the glow discharge has been also known. However, the glow discharge has a problem that controllability thereof is low. In general, the arc discharge has been dominantly applied to a welding process, a cutting process, etc.
The arc discharge has been used to melt or fuse an object to be processed by utilizing its high electrode temperature and its high positive column temperature (gas temperature). However, use of the arc discharge causes a problem that no processing for the object can be carried out without heating the object, for example, at a room temperature because the temperature of the object to be processed is increased to 2000 to 6000K in the arc discharge.
In order to enable fundamental treatment, processing, etc. at a room temperature, an attempt to stably induce the glow discharge under atmospheric pressure has been made (by S. Kanazawa, et. al., J. Phys. D; Appl. Phys. 21 (1988) pp 838-840). The following features have been known as necessary conditions to stably perform the glow discharge under the atmospheric pressure: (1) a discharge space is filled with He, (2) an insulator is inserted between electrodes (in a discharge path), (3) at least one of the electrodes is shaped in a needle or brush form, and (4) frequency of applied electric field is above 3 kHz. The insulator is used to prevent the discharge from being shifted to the arc discharge, the setting of the frequency of the applied electric field above 3 kHz is used to make current flow through the insulator, and the needle or brush shape of the electrode is adopted to make the electric field ununiform so that the discharge is liable to be induced. Various treatments such as an etching treatment for etching the surface of organic material such as polyimide, etc., or the surface of inorganic material such as silicon, etc., have been attempted with the above method. However, any one of these methods requires a process for reducing the pressure of an reaction space to vacuum once and then filling gas such as helium into the space although they are carried out under atmospheric pressure. In addition, the treatment for a substrate is uniformly carried out over the surface of the substrate, and thus it is impossible to locally and selectively treat a minute area of the surface.
In order to overcome the above disadvantage, the inventor of this application invented a plasma generating apparatus in which stable low-temperature plasma is formed in an atmospheric-pressure state of open system using no process of reducing the pressure of the system to vacuum once, and the plasma thus formed is enclosed in a minute area to subject the minute area to a processing, a surface treatment and an etching, and an etching method using the apparatus (as disclosed in Japanese Patent Application No. 2-286883).
In the plasma generating apparatus and the etching method as described above, electrodes comprising conductors which are formed of metal or the like are concentrically and cylindrically arranged, and a cylindrical insulator is inserted into a gap between the electrodes so as to be concentrical with the electrodes and in contact with the outside electrode. A helium mainly-contained gas is kept to flow through a gap between the insulator and the center electrode under atmospheric pressure, and alternating current is supplied between the electrodes to ionize the helium mainly-contained gas, whereby plasma is generated in the gap between the insulator and the center electrode. An etching gas is introduced into the plasma to perform an etching treatment.
According to this method, low-temperature plasma can be obtained with a simple experimental apparatus, and an etching processing can be carried out even for organic material having no heat resistance using halogen compound gas as etching gas. In addition, the temperature of plasma is confirmed to be below 100.degree. C. with a thermocouple when an applied alternating electric field (13.56 MHz) is about 100 W.
The plasma generating apparatus under atmospheric-pressure condition in an open system has been researched and developed by the inventor of this application as described above, so that practical use of low-temperature plasma under atmospheric pressure would be expectable. However, this method still has various problems to be solved. One of the problems is a problem of a plasma discharge area, that is, an area to be treated by plasma (plasma-treatment area) which is exposed to outside air.
The low-temperature plasma discharge modes under atmospheric-pressure condition, containing the above invention, which have been conventionally proposed are mainly classified into the following two types.
1. Types to which a parallel and planar type discharge apparatus is applied PA0 2. Types using needle-shaped/beam-shaped plasma
Some apparatuses were proposed in the past. The basic construction is as follows. A discharge space is formed by two parallel electrodes one of which is provided with a dielectric plate, and these electrodes are accommodated in a discharge chamber which is kept under atmospheric-pressure state by plasma source gas to perform an atmospheric-pressure discharge. A sample to be subjected to a film formation treatment or an etching processing is mounted on the electrode. The plasma exists at only a space between the opposed electrodes.
Representative one of these types is the apparatus as described above, which was invented by the inventor of this application. That is, a needle-shaped electrode is disposed at the center of the void of the cylindrically-arranged electrodes, and the discharge is carried out under atmospheric-pressure state. A beam-shaped plasma is obtained not only at the void of the electrodes, but also by gas flow.
The above two apparatuses are unpracticable from the viewpoint of the discharge area. In the conventional parallel and planar type, the discharge is carried out in a non-open system inside of a chamber, and thus a chamber and an evacuation device are required. In addition, it is unpractical from the viewpoint of the application of the plasma to the film formation/processing, etc. in a completely-open system which has been conventionally attempted by the inventor of this application. Further, the accommodation of the apparatus with a sample into the chamber deteriorates flexibility even though the discharge area is large.
It was proposed in the past that the film formation/processing in an open system was carried out using a plasma generating apparatus for generating a needle-shaped/beam-shaped plasma. In this method, the needle-shaped/beam-shaped plasma generating apparatuses were arranged in series to obtain a discharge state where an approximately sheet-shaped plasma is obtained. However, in this method, radical density is extremely different between the central portion and the outside portion in the needle-shaped/beam-shaped plasma, and this difference appears as fluctuation of radical density in a sheet in accordance with a position of the electrode. Therefore, an uniform sheet-shaped plasma has not been hitherto obtainable under atmospheric-pressure state in an open system.
In addition, the plasma under atmospheric pressure has high collision possibility between particles because of the atmospheric pressure (that is, mean free path is short), so that the possibility of electron-ion recombination in a space becomes high. As a result, the plasma is not spatially broadened, and it is rapidly extinguished as the potential gradient is lowered, so that there occurs a problem that the number of radicals which arrive at a substrate is extremely reduced. In order to prevent this reduction, the substrate may be approached to the discharge area as nearly as possible. However, the radical density is greatly varied in accordance with slight variation of distance in the neighborhood of the discharge area, so that there occurs another problem that the controllability of the radical density is remarkably deteriorated.
The conventional etching treatment method using the glow discharge under atmospheric pressure has a problem that an object to be processed is charged when the object is formed of insulator. Charges to be supplied to the object mainly contain electrons, and thus the supply of the electrons to the object would be disturbed if the object is charged. Consequently, the number of the radicals to be supplied to the object together with the electrons is also reduced, and thus the etching efficiency has been lowered.
As described in the conventional method, the use of the helium mainly-contained gas is indispensable to generate low-temperature plasma under atmospheric pressure. For example, when gas such as argon is used in place of the above gas, electric power above 100 W must be supplied, and thus there occurs a trouble that the central electrode is melted in a long-term discharge operation.
The cause of such a phenomenon has not been hitherto elucidated. However, from the fact that the discharge could be performed with argon by increasing electric power as described above, it is considered that simple substance of argon gas can provide a coexistent state of low-temperature plasma and fine arc discharge, and the fine arc discharge induces a thermal obstruction to an apparatus.
As a countermeasure to such a thermal obstruction in the atmospheric-pressure discharge using argon gas, success of discharge by adding argon with ketone (acetone) was reported. However, in this reported method, it is impossible to perform discharge under atmosphere having no additive. That is, the discharge using only argon gas was not succeeded in this method.